This invention concerns the collection of solar energy using a particular type of electroplated and treated surface.
Although most black surfaces are efficient absorbers of solar radiation, many such as ordinary black paints are also efficient radiant emitters of thermal energy at temperatures to which they might be heated by solar absorption such as 90.degree. to 250.degree. C. The net result is that much of the energy absorbed is lost as heat radiated back out into the atmosphere. It is desirable to have a surface which will absorb a maximum of solar radiation in the visible, ultraviolet and infrared portions of the spectrum and emit a minimum of radiation at the temperature of the surface under black body conditions, which would be mainly infrared radiation.
Solar energy absorbers can be used in many types of heat exchange devices. A typical device would involve passing the solar radiation through two transparent windows with a dead air space between and then absorbing it on a black surface of a flat piece of metal, known as a flat plate collector. Water conduits would be built into or attached to the back of the piece of metal, with insulation behind the conduits. The conduits would take the heated water to a location where the heat could be extracted, permitting recirculation of the water. The insulation minimizes heat loss out the back of the device. The double window minimizes conductive heat loss through the window following convective heat transfer by air in the device from the absorber to the window.
The final substantial cause of heat loss in such a device is thermal radiation from the absorber through the double window. While windows can be provided which are more transparent to incoming solar energy than to outgoing thermal energy, such as with interference filters, it is desirable to have available an absorber with substantial solar selectivity.
Solar selective coatings are known. Some of them are produced entirely by electroplating, which can have the disadvantage of uneven plating from one part of the surface to another. Others are more expensive or complicated in their preparation than desirable.
Aside from solar selectivity, it is known that black coatings can be produced by electroplating of several metals for decorative purposes or to minimize glare, such as on weapons. Black chromium and black nickel electroplates are known. A white brass electroplate blackened by dipping in weak nitric acid is taught in Canadian Pat. No. 444,509 -- Byers (1949) and by B. Pushpavanam et al. in "Black Finishes on Zinc", Electroplating and Metal Finishing, December, 1974, p. 9, 11; and it is known that copper-contaminated zinc electroplates are susceptible to blackening by nitric acid. The original white brass electroplating patent, U.S. Pat. No. 2,221,562 -- Wernlund (1940) teaches the production of white brass containing 19 to 31% copper, balance zinc.
However, solar selectivity of blackened white brass is not gnerally known.